The Problem of Blade-Passing Noise
Let it be assumed that the impeller of a fan has a number z of blades that are arranged at identical distances (“equidistantly”) in a circumferential direction, and that the impeller rotates at a rotation speed n (n being measured in revolutions/sec or s−1).
What then results is a pressure fluctuation that is perceived by a stationary observer at a frequencyBPF=z*n  (1)This is referred to as “blade-passing noise” or a “blade-passing frequency” (BPF).
This blade-passing noise has an unfavorable effect on the acoustic quality of a fan because:
it represents an isolated energy-rich maximum in the acoustic power spectrum, and
it is usually located in a frequency range to which human hearing is particularly sensitive.
The acoustic quality of fans plays an important role especially in workplaces or, for example, for a decentralized climate control system. Optimization thereof, and thus a reduction in the blade passing noise, is therefore very important.
Variable Angle Distribution
A well-established method for reducing blade-passing noise is variable angle distribution between the blades. The result is to distribute the blade-passing noise over several frequencies that differ slightly from one another. This can have psychoacoustic advantages because the radiated acoustic energy is no longer concentrated at a single frequency, and can therefore also be utilized in the context of the present invention.